Fractionation and Characterization of Alkali-Soluble Lignins from Wheat Straw

Abstract The chemical compositions of seven alkali-soluble lignin fractions isolated from partly dewaxed wheat straw with sodium hydroxide at successively increasing strength from 0.125 to 1.50 M are reported. The physico-chemical properties of the lignin fractions were characterized by using GPC-, UV-, IR-, 13C-NMR- spectroscopy and alkaline nitrobenzene oxidation. No significant difference in monomeric composition was found in the case of wheat straw lignin fractions. However, more condensed fractions were found to be LA 4-7 and less condensed fractions appeared between fractions LA-1 and LA-3. A method of two steps of precipitation was proposed in this study to obtain the lignin fractions relatively free of polysaccharides. The isolated lignin fractions contained rather low amount of polysaccharides (1.7–1.9%), and had low molecular-average weight between 1281 and 1796. It was found that p-coumaric acid is ester-linked to lignin at C-α and C-γ, while most of the ferulic acids are ether-linked to lignin at C-α and C-γ.